It is generally known to round intersections of bores in high pressure-resistant components, in particular components of fuel injection systems, in order to counteract any local stress peaks which can lead to component fatigue or destruction.
In a rounding process which is also known as extrude honing, a polymeric paste mixed with abrasive particles is forced through the bores. This breaks burrs and rounds the cut edges. Drawbacks of this process are high running costs, on account of the need to purchase and dispose of the polymeric abrasive paste, and a very expensive cleaning process required to remove the abrasive paste from the component. Moreover, in fuel injection systems there is a risk of the paste being entrained, for example downstream to the nozzle. This can lead to blockages of spray holes in the nozzle or to the loss of the sealing function of the nozzle in the region of the nozzle needle, and therefore ultimately to a loss of power, failure of the engine or even engine damage.
A further possible way of rounding edges which is generally known in the prior art consists in using an electrochemical material-removal process. In this case, the edge is likewise rounded in the region of the intersecting bores. Drawbacks of this process are in particular the pore-like rough surface which is formed and the corrosive damage to the grain boundaries of the material, which lead to stress peaks in the microscopic range. Consequently, the increase in pressure which can be achieved with this process is lower than in the extrude honing process.
It is also known to introduce a compressive stress in the inner walls of the bores and passages by grinding or honing in order to increase the compressive strength of the component. After the grinding or honing, internal compressive stresses are formed in the inner wall of the bore or the inner region of the passage. This internal compressive stress counteracts the tensile stress generated as a result of the internal hydraulic pressure.
Furthermore, German patent application DE 199 53 131 A1 has disclosed a method and an apparatus for rounding edges in components which are subject to high mechanical, thermal or other loads. One particular application area which is mentioned is the rounding of edges at intersections of passages in high-pressure reservoirs of fuel injection systems. In such highly loaded components, stress peaks occur in all kinds of edges and can lead to component failure, in particular to component fracture. To make the component resistant to high pressure, its edges are rounded. The rounding is effected by causing an erosive liquid, which is passed through the component by a delivery pump, to flow over the edge which is to be rounded. In the region of the edge, the flow velocity of the liquid is increased by means of a cross-sectional narrowing, in order to enhance the erosive action of the liquid. The flow velocity of the liquid and therefore also the amount of material removed in the region of the edge can be influenced by setting the delivery pressure. The delivery pressures are approximately in the range from 50 bar to 140 bar. Moreover, it is stated in general terms, without any further details being provided, that the main direction of flow of the liquid and the longitudinal axis of the edge which is to be rounded preferably include an angle of 90°. To round the sharp-edged transition between a nozzle needle seat and an adjoining antechamber before the injection holes of an injection nozzle, this document describes introducing a conical body into the region of the nozzle needle seat of the injection nozzle, which is in the form of a blind bore, in such a way that an annular gap is formed in the region of the edge. This annular gap serves to achieve the desired increase in the flow velocity in the region of the edge that is to be rounded.
Also, a further apparatus for hydro-erosively rounding an inlet edge of a spray hole in an injection nozzle for fuel is already known from German patent DE 199 14 719 C2. Unlike the rounding apparatus described above with a conical flow body for increasing the flow velocity of the erosive liquid, in this case there is a flow body which resembles the shape of a nozzle needle. In addition, guide grooves running in the longitudinal direction of the tip of the flow body in the shape of a nozzle needle can be machined in the outer wall of this tip and can be used to deliberately guide the abrasive bodies in the erosive liquid onto the upper region of the inlet edge of the spray hole. The intention here is to increase the rounding in this region, which should then lead to a higher through-flow velocity for the fuel.